The field of the present invention relates to control systems for transport refrigeration systems. More specifically, the present invention is directed towards controlling the generator power requirements through the combined control of the suction modulation valve (the xe2x80x9cSMVxe2x80x9d) the compressor cylinder unloaders, the speed of the diesel engine for the system, and the superheat setting of the expansion valve.
A common problem with transporting perishable items is that often such items must be maintained within strict temperature limits, regardless of potentially extreme operating conditions required by a high ambient temperature and/or other factors. These extreme conditions can cause an excessive power draw from the diesel engine powering the system, thus potentially causing unwanted system shutdowns or even adversely impacting the useful life of the engine. Recent inventions by the assignee of the present application have enabled significant cost savings through the implementation of an electrically driven trailer refrigeration unit from a synchronous permanent magnet generator. However, the use of this new system has a disadvantage of significant power supply limitations compared to prior art devices. Thus, there is a need for an efficient controller which optimizes power management for the draw placed upon the generators of such transport systems.
Currently available controller designs sold by assignee disclose the use of suction modulation valves (xe2x80x9cSMVsxe2x80x9d) to limit the maximum system current draw. In addition, such units use a suction modulation valve (SMV) to limit the maximum system current draw, but not to control the voltage. The SMVs of such systems close quickly, but result in pressure drop problems which limit peak capacity problems, and create reliability and efficiency issues. In addition, previously available prior art controls are comparatively crude to that needed for newer, power limited systems, which call for sophisticated, combined controls that monitor and manipulate superheat settings, compressor cylinder unloaders, and engine speed in order to prevent unacceptable power draw on the transport refrigeration system. The inventors of the system and process disclosed in the present application have found significant improvements in generator power management by controlling just such parameters, thus decreasing system component wear and tear and increasing the engine and generator life.
The apparatus and control method of this invention provides a refrigeration unit for a transport system having a controller which monitors and prevents power draw overload conditions for the generator in situ. For example, the algorithm in the controller is designed to adjust for changes in box temperature and speed changes. The present invention manages power (indirectly) by monitoring and controlling engine speed and current. The controller of the present system is designed to avoid overpowering the generator and the engine (i.e., just to maintain the generator and the engine unit running), and is further designed to avoid shut down (and potentially, damage). Such conditions are avoided by staying below a preselected maximum system power limit rating.
The present invention does not maximize capacity in in the box or container of the transport refrigeration system, the present invention is rather directed towards minimizing the limits place upon current levelxe2x80x94thus, the present invention seeks minimize the reduction of the optimal refrigeration capacity level. Power, and consequently current draw, rise and fall with mass flow. Thus, the controller of the present invention monitoring current and controls mass flow as a means to control power draw on the generator.
Also, the present invention seeks to limit the temperature of the generator by controlling the SMV. If the generator temperature for a permanent magnet rotor exceeds a certain point it will demagnitize and thus require an expensive and time consuming overhaul. Thus, the controller of the present unit monitors generator temperature and restricts mass flow in the system (thus decreasing power draw in the event the generator temperature is above a preselected limit. Specifically, the generator temperature sensed goes above a xe2x80x9csoftxe2x80x9d limit, the controller further restricts the SMV (indirectly, by reducing the maximum allowable current, through the PID control in the processor). If the generator temperature sensed goes above a xe2x80x9chardxe2x80x9d limit, then an alarm is issued, and the unit may initiate shut down.
Finally, the present invention seeks to minimize or eliminate the step function caused by the deenergizing of each loader of the system compressor, which load two additional cylinders in the compressor. Each deenergized unloader, when deenergized, has the effect of increasing mass flow at least 50%, and can increase current beyond the maximum current draw permitted. Thus, the present invention, using programming in the microprocessor of the controller, increases the superheat setting, which results in restricting the electronic expansion valve (the xe2x80x9cEXVxe2x80x9d) and thus reduces the mass flow prior to the unloader being deenergized. The superheat setting (and thus the EXV) is then gradually reduced to its initial base levels once the current draw sensed is below a preselected limit.